The video illustrates your point very well.
Mine I think would be medium/soft - compared with the video above - I am able to swim with less knee bend and less 'snap'. I can make slower undulations which provide thrust over a longer period of time and with less effort. It is the most organic feeling monofin I have used. (my others are all stiffer) I am also able to swim long distances on the surface very comfortably (blade angle I think has much to do with this.)
Mine I think would be medium/soft - compared with the video above - I am able to swim with less knee bend and less 'snap'. I can make slower undulations which provide thrust over a longer period of time and with less effort. It is the most organic feeling monofin I have used. (my others are all stiffer) I am also able to swim long distances on the surface very comfortably (blade angle I think has much to do with this.)